1. Field of the Invention
The present invention relates generally to a spectrometer lens having a magnetic lens, a deflection unit, and a retarding field spectrometer for particle beam measurements through analysis of secondary particles.
2. Description of the Related Art
Conventional scanning electron microscopes which are equipped with beam blanking systems and retarding field spectrometers are currently used for quantitative measurements of potential at nodes and interconnects in LSI (large scale integrated) circuits. However, it is not possible to generate sufficiently fine electron probes for investigating VLSI (very large scale integrated) circuits having structures in the submicrometer range using modified scanning electron microscopes, since the electron microscopes must be operated at low accelerating voltages to avoid radiation damage as well as charging of the components which are usually mounted on non-conductive or poorly conductive carrier substances.
Spatial resolution by the known scanning electron microscopes is essentially limited by the axial chromatic aberration of the objective lens and by the electron-electron interaction, or Boersch effect. A clear improvement of the spatial resolution can be achieved only by using a short electron-optical beam path having few beam cross overs and by using an objective lens of short focal length. The use of short focal length objective lenses to reduce chromatic and spherical aberrations essentially defined by the focal length have hithereto failed due to the structure of conventional electron beam measuring units which include a secondary electron spectrometer disposed between the objective lens and the specimen under test.
Only due to the development of objective lenses having integrated secondary electron spectrometers, also known as spectrometer-objectives, could the aberration of the objective lens and, thus, the probe diameter at the specimen be reduced together with the working distance. The spectrometer-objective is known from the publication of Kawamoto "Electron Beam Tester with in the Lens Analyzer" which appear in the Proceedings of the Symposium on Electron-Beam-Testing, Nov. 9-10, 1984, Osaka, Japan, pages 69-72. The disclosed arrangement includes a magnetic objective lens having a short focal length in which is integrated a parallel plate analyzer composed of extraction and retarding field electrodes. It is not possible, however, with the disclosed spectrometer-objective to document angle-independent secondary electrons triggered at the specimen and emitted in a greater solid angle range. Thus, measuring errors result which limit the possible resolution of potential.
A spectrometer-objective comprising an integrated magnetic deflection system is disclosed in German Patent Application No. P 35 21 464.3 and corresponding U.S. patent application Ser. No. 874,498, filed June 16, 1986. For the disclosed arrangement, the deflection of the primary electron beam is within the spectrometer-objective so that the space required in conventional scanning electron microscopes for two-stage deflection systems between the condenser lens and the objective lens is eliminated, and the influence of the lateral Boersch effect on the probe diameter is reduced. Since both the primary electrons and the secondary electrons which are accelerated in the direction of the retarding field electrode are deflected in the field of the integrated deflection unit, an angle-independent documentation is generally possible only for particles emitted in the direction of the symmetry axis of the secondary electron lobe.